The invention relates to an isolation monitor comprising :
an input circuit of a reference AC signal between the power system and earth, PA1 an electronic detection circuit supplying a measurement signal which is the image of the power system isolation, said isolation corresponding to an impedance Zd formed by a leakage resistor Rf electrically connected in parallel with a stray capacitor Cf, the capacitance of which depends on the characteristics of the power system, PA1 discrimination means of the resistive and capacitive components of the measurement signal, PA1 and computation and indication means of the modules of said components to display the values of the leakage resistance Rf and of the stray capacitor Cf on display and alarm means. PA1 a keyboard with control keys to enter the data and scroll the display, PA1 a first alphanumeric read-out to display the values of the leakage resistance Rf, the stray capacitor Cf, and the earth resistance Rt, PA1 a second bar graph read-out to give permanent indication of the isolation level with respect to alarm and pre-alarm thresholds, PA1 an alarm device with signalling relays supplied in the event of said thresholds being exceeded, PA1 and a communication interface with a bus to transmit or receive data from a external transceiver device.
The document FR-A 2,616,228 describes an isolation monitor containing a generator which applies a low-frequency reference AC voltage to the power system, and a synchronous analog detector having a pair of solid-state inverters processing the resistive and capacitive components of the measurement signal proportional to the isolation impedance. A converter is connected to the generator to deliver two control signals in quadrature to control the inverters, one of the control signals being in phase with the reference AC signal applied to the power system by a generator. The precision of the monitor depends on the phase stability of the measurement channel. The use of a generator with a synchronous analog detector is liable to make the phase distortion factor vary according to the characteristics of the electronic components, which gives rise to measurement errors.
The object of the invention consists in improving the measurement precision of a isolation monitor.